In a scanning optical apparatus for scanning a target surface with a beam of light emitted from a light source using a polygon mirror deflecting the beam of light, the position in which the beam of light strikes each reflecting surface (each of specular surfaces of the polygon mirror) would vary according to a deflection direction in which the beam of light reflected therefrom travels. This phenomenon is called “sag”. The sag would be a cause of curvature of field or other kind of degradation of images on the target surface.
To reduce such adverse effects of the sag, the position in which the beam of light from the light source strikes each reflecting surface (i.e., the distance between the center of rotation of the polygon mirror and the optical axis of an optical system from which the beam of light from the light source comes to the polygon mirror) may be adjusted within a predetermined range of values such that the sag is rendered nearly symmetric with respect to the reference deflected chief ray.